Modern digital communication systems rely on transmitters and receivers that are synchronized with each other. In a cellular radio telephone system, user equipments (UEs), such as mobile phones and other remote terminals, are synchronized with network base stations (BSs), which can also be called nodeBs. Digital communication systems include time-division multiple access (TDMA) systems, such as cellular radio telephone systems that comply with the GSM telecommunication standard and its enhancements like GSM/EDGE, and code-division multiple access (CDMA) systems, such as cellular radio telephone systems that comply with the IS-95, cdma2000, and wideband CDMA (WCDMA) telecommunication standards. Long Term Evolution (LTE) can be seen as an evolution of the current WCDMA standard. Digital communication systems also include “blended” TDMA and CDMA systems, such as cellular radio telephone systems that comply with the universal mobile telecommunications system (UMTS) standard, which specifies a third generation (3G) mobile system being developed by the European Telecommunications Standards Institute (ETSI) within the International Telecommunication Union's (ITU's) IMT-2000 framework. The Third Generation Partnership Project (3GPP) promulgates the UMTS, LTE, WCDMA, and GSM standards, and specifications that standardize other kinds of cellular radio communication systems.
A BS transmits frequency correction and synchronization signals that enable a UE to synchronize itself to the BS and if necessary to correct the UE's internal frequency standard to be in line with that of the BS. The BS mainly providing service to a UE is usually called the UE's “serving” BS or cell. The timings of signal frames and time slots, which organize information conveyed between a UE and BS, are related to a common set of counters that run continuously whether the UE and BS are transmitting or not, and so after a UE has determined the correct setting of these counters, all its processes are synchronized to its current serving BS.
Characteristics of physical and transport channels (Layer 1) in the frequency-division-duplex (FDD) mode of a WCDMA cellular radio communication system are defined in 3GPP TS 25.211 V8.4.0, Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD) (Release 8) (March 2009), among other specifications. In general, transport channels are services offered by Layer 1 (L1) to higher layers according to the OSI model and are defined by how data is transferred over the air interface between a BS and a UE. Dedicated channels use inherent addressing of UEs, and each of successive radio frames consists of fifteen time slots, with the length of a slot corresponding to 2560 chips, or ⅔ millisecond (ms). Each frame is also organized into successive subframes, each consisting of three slots, with the length of a subframe corresponding to 7680 chips, or 2 ms. A WCDMA communication system is described here, but it will be appreciated that other systems have equivalent features.
A UE times its transmissions to a BS in line with those received from the BS. The BS sends to each UE a timing advance (TA) parameter, which can be quantized as a number of symbols, that is based on the perceived round-trip signal propagation delay BS-UE-BS, or equivalently UE-BS-UE. The UE advances its transmission timing by the TA with the result that a signal transmitted by the UE arrives at the BS compensated for the propagation delay, i.e., at a time approximately independent of the distance between the BS and UE. Basically, this means that the UE is configured to transmit data earlier in time when it is far away from the BS compared to when it is close to the BS.
FIG. 1 is a plot of TA and transport block (TB) size as functions of distance between a UE and a BS. A transport block can be considered the basic data unit exchanged between L1 and medium access control (MAC) entities, or signal processing devices, in a UE, and the TB size is just the number of bits in a TB. The MAC layer is a sub-layer of Layer 2 (L2) that provides unacknowledged data transfer service on logical channels and access to transport channels, i.e., L1. FIG. 1 depicts, in a conceptual way, that as the distance to a BS increases, the TA increases, and the TB size decreases. From the UE's point of view, the time offset between the signal it transmits on the uplink (UL) to the BS and the signal it receives on the downlink (DL) from the BS changes as the TA changes.
In a communication system like a cellular telephone system, a UE processes its received signal in real time, which is to say that the digital processing procedures applied to each symbol or group of symbols of a received signal keep pace with the arrival of successive symbols or groups. Thus, in a real-time system, it is important to know the maximum runtime for each procedure in order to be able to set and evaluate deadlines for tasks that combine several procedures. For example, the electronic processing blocks in a UE that produce the baseband signal have to complete their processing by a specific deadline so that the baseband signal can be impressed on a carrier signal and the modulated carrier can be transmitted out on the UE's antenna at the right time. Some approaches to easing the time pressure on a UE are known. For example,
U.S. Patent Application Publication No. US 2009/0122731 describes partially discarding a cyclic prefix of a last-received OFDM symbol as a technique for completing data reception earlier and switching to data transmission in a timely manner.
Because the TA, which is controlled by the serving BS, causes a UE to transmit data earlier when it is far away from the BS and later when it is close to the BS, the UE must take the TA into account in determining its processing deadlines. A larger TA leaves the UE with a smaller time period for processing its UL signal. For example, the time between a UE's receipt in the DL of an UL transmission grant and the time the UL data must be sent is greater for a smaller TA than it is for a larger TA, and as depicted in FIG. 1, the maximal bit rate in the UL (and the DL) decreases as the distance between the BA and UE increases. Many current communication system specifications do not define any relationship between TA and available UL bit rate.